12. The effect of Bracing Arm with Extracoronal Attachment use evaluated by Three Dimensional Finite Element Method

Kanbara R., Ando A., Nakamura Y., Kumano H., Miyata T., Masuda T., Ohno Y. and Tanaka Y.

Removable Prosthodontics, School of Dentistry, Aichi-Gakuin University

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Introduction

In unilateral extension partial denture treatment situations, magnetic attachments are often used as a hidden retaining retentive elements to provide a superior esthetic result and improved patient comfort. When an extracoronal magnetic attachment is used for retention, the mechanics of a functional cantilever effect of denture behavior should be well understood^1-3.

　Solutions to account for cantilever increased stress transfer concerns include the use of splinting abutments, use of lingual bracing arms and extracoronal interlocking attachments. There are few reports or evidence-based evaluations regarding the relative use and merits of these different design elements for improved treatment decision making purposes.

Objective

The aim of the present study was to investigate the effect of bracing arms and interlock designs employed with extracoronal attachments as applied to unilateral partial denture designs using the three-dimensional finite element method (FEM), and to seek alternative designs and comparisons.

Materials and Methods

Fig. 1 shows the overview of the three-dimensional FEM model. The three-dimensional FEM model was constructed from the CT data of patients according to the method developed by Ando. (Ando A, 2009, Aichi Gakuin)

Dentures, crowns and retainers were fabricated on the study model using Scanning Resin (Yamahachi Dental Mfg., Co.), and were built into the Ando analysis model. (Ando A, 2009,Aichi Gakuin)

Table 1 shows components and material constants of the model. The same property value was set for crowns, attachments, metal frames and retainers. A preliminary experiment was performed before setting material constants so that the vertical displacement of teeth against the applied load is close to the known value. As for the constraining conditions, the bilateral coronoid processes was a complete restraint. A total of 10 N vertical loads were applied on the occlusal surface (Fig. 2).

The analysis model included connective crowns of #3, 4 and 5 based on Ando's results. Slits were prepared on the distal parts of extracoronal attachments in all models (Fig. 3).

The following are four analysis models fabricated in the present study (Fig. 4).

1. The model with a bracing arm and an interlock incorporated into an extracoronal attachment (B-A model).

2. B-A model without a bracing arm (B-A less model).

3. B-A model with medial rests on the teeth # 4 and 5 (Rest model).

4. Lingual bar model with a twin clasp placed on the opposite side as an indirect retainer (L-B model).

Contact conditions were introduced between a denture and the mucosa, and an abutment and an attachment. The coulomb friction was applied, and friction coefficient was set at 0.01.

Results

1. Stress distribution

The Von Mises stress was used to analyze the stress distribution.

1) Connective crown

Fig. 5 shows the stress distribution in connective crowns where the load is directly transferred to the denture. For all four models, stress concentration was observed in attachments, #4-5, and #3-4 interconnected areas. In the B-A less model, the highest stress concentration was observed in #4-5 interconnected area. The highest stress relaxation was observed in the L-B model. The Rest model showed different results from other three models, and the stress concentration was also observed in #4-5 and #3-4 connective areas.

2) Attachment

Fig. 6 illustrates the stress distribution of attachments. Stress concentration was observed in the upper and neck parts of an attachment in all models. Stress distribution patterns were similar between the B-A and Rest models.

The stress value was calculated at the measuring point set in the neck part where the fracture is more likely to occur. The graph in Fig. 7 shows the stress at the neck part of extracoronal attachment. The B-A less model demonstrated the highest stress value, and the L-B model demonstrated the highest stress relaxation in the neck part. The comparison between the B-A and Rest models that have similar stress distributions revealed a higher stress value in the Rest model.

3) Abutment

Fig. 8 shows the stress distribution of abutment teeth #3, 4 and 5. Stress concentrations were observed at the mesiodistal margin of the tooth #5, and the distal margin of the tooth #4 in all models. No stress concentration was observed around the margin of the tooth #3. For the stress distribution of the root, the L-B model showed the highest stress relaxation. There was no significant difference in stress distribution in other three models with unilateral design. The stress value was calculated at the measuring point set in the cervical area of the most-posterior tooth which is most affected by any denture cantilever effect.

The graph in Fig. 9 shows stress at the distal cervical margin of the tooth #5. The B-A less model demonstrated the highest stress value, and the L-B model demonstrated the highest stress relaxation. The comparison between the B-A and Rest models revealed a slightly higher stress value in the Rest model.

2. Displacement

The coordinate axes were established in the abutment and the posterior margin of the denture, and the displacement amount was measured. Fig. 10 and 11 showed the distal displacement of the abutment, and the vertical displacement of the posterior margin of the denture, respectively.

1) Abutment

The L-B model showed the smallest distal displacement of the abutment. Other three models with unilateral design demonstrated large distal displacement, notably the B-A less model. No significant difference was found between the B-A and the Rest models.

2) Denture

The L-B model showed the smallest vertical displacement of the denture. Other three models with unilateral design demonstrated large vertical displacement, notably the B-A less model.

Discussions

1. Analysis model

Simple analysis models have been used in the previous studies due to the complexity of model construction. Although there was a consistency in the analysis results obtained from these models, these results were not reliable enough to use as the clinical evidence due to their simple design. The model construction method developed by Ando enables to construct the finite element model with a realistic size and structure in a non-invasive manner by simulating different restorations. In the present study, the effect and availability of a bracing arm and interlock employed with an extracoronal attachment in unilateral partial dentures were examined, and a new model with the same effect and requires less laboratory work was constructed. The finite element model was used as a fundamental model.

2. Analysis results

The analysis of stress distributions and vertical displacement of a denture demonstrated the following results. Although the vertical displacement of a denture with the attachment and the distal margin of the second premolar were larger in the B-A model than the Rest model, the stress value was smaller in the B-A model. This observation is due to the differential displacement of a denture when the load is applied. The Rest model revolved buccal direction around the upper area of the extracoronal attachment in the denture. On the other hand, the B-A model revolved around the interlock area at the tip of a bracing arm. The difference between these two models is considered to contribute to stress mitigation findings. The stress distribution figure (Fig. 5) shows the stress concentration in the connective area between the first and second premolars both in the B-A and B-A less models. The result suggested the stress concentration is centered around an interlock when the load is applied.

These results showed that a bracing arm mitigates and distributes the stress on an extracoronal attachment, and distal margin of the posterior abutment tooth.

Conclusions

The effect and availability of a bracing arm and interlock used with extracoronal attachments was examined in this study. The following conclusions were drawn:

1. In unilateral free-end edentulous case, the L-B model is considered to be an optimal design based upon the analysis results of the stress distribution and displacement findings.

2. Bracing arms proved to be effective in an unilateral denture design.

3. The analysis result of the Rest model was similar to the B-A model, suggesting that mesial rests can replace a bracing arm.

Discussion Board